This invention relates to a method and apparatus for compensating for the IR drop which can occur between electrodes in an electrochemical cell when it is subjected to a current field other than that which results from the measurement process alone. This invention is particularly useful in compensating for the IR drop which occurs when an electrochemical cell is recalibrated by the brief application of a d.c. current to a supplementary electrode as set forth in a U.S. patent application Ser. No. 495,302 filed on the same date as this application described below.
The description of the electrochemical cells disclosed in U.S. Pat. No. 4,076,596 is hereby incorporated by reference.
As set forth in the last mentioned U.S. patent, the basic polarographic apparatus as improved by Clark is described in U.S. Pat. No. 2,013,386 and is known as the Clark cell. The Clark cell utilizes a dual electrode structure immersed in an electrolyte and encased in a membrane which is permeable to the species to be measured. Typically, when used for oxygen analysis the cathode in the Clark cell is formed of platinum or gold and is located closely adjacent to the membrane while the anode is formed of silver or lead, with an electrolyte usually made of an aqueous alkaline halide solution. In operation, the Clark cells is characterized by the fact that the cell consumes the species being measured and therefore causes a depletion of the species from the fluid sample in which the cell is immersed.
To avoid disadvantages such as stirring dependencies, a cell structure of the type shown in U.S. Pat. No. 3,260,656 was proposed by Ross. That cell utilized an electrode system which consumed the species being measured at one electrode which generated a like quantity of the species at the electrode of opposite polarity. The electrodes were closely spaced so as to avoid depletion of the species from the sample.
The Ross type cell was further improved by the Connery et al cell structure of U.S. Pat. No. 4,076,596. For the purpose of this description, those cells which characteristically deplete the sample of the species being measured are referred to as Clark-type cells, or cells which operate in the Clark mode, whereas those cells which do not deplete the sample, such as the Ross or Connery et al. cells, are herein referred to generically as the Ross-type cells or cells operating in the Ross mode.
Electrochemical cells of the Ross type as described in U.S. Pat. No. 4,076,596 have been found to be very useful in applications where it is necessary to sterilize the cell before it is used in order to prevent contamination of the sample to be measured. In such applications and in others, it is not possible for the cell must be withdrawn from the sample and inserted into a known but possibly contaminated standard sample for recalibration. During the course of normal use these cells require recalibration because of the normal cell drift which takes place.
In the related application referenced above, certain coworkers of mine have disclosed a method for recalibrating electrochemical cells of the Ross type in situ, by applying a d.c. current to a supplementary electrode in the cell and measuring the change in cathode current which results. That change is then related to the change which occurred under similar conditions at the time of a previous or initial calibration as an indication of the change in calibration which has occurred.
This method of recalibration has the disadvantage that it creates certain small errors as a result of the additional current introduced into the cell. These errors are due to an interelectrode IR drop which results from that additional current, and manual adjustment of the bias for the cell electrode to compensate for those errors is a tedious and time consuming procedure.
It is an object of this invention to provide a method and apparatus for automatically compensating for the IR drop introduced by recalibration or by other sources of additional current in the cell.